Separator for an alkaline storage cell

ABSTRACT

A separator for an alkaline storage cell comprises: two sheets of felt (10, 20) made of polypropylene, polyamide, or polyethylene, and having a thickness lying in the range 0.1 mm to 0.4 mm; and an intermediate sheet (4) that is plane, microporous, and made of a sintered material selected from polyvinyl chloride and polyethylene. The thickness of said microporous sheet lies in the range 0.1 mm to 0.4 mm; the diameter of its pores lies in the range 10 μm to 100 μm, and its weight lies in the range 100 g/cm 2  to 400 g/cm 2 .

The present invention relates to a separator for an alkaline storagecell, in particular a nickel-cadmium storage cell in which theelectrodes may have a sintered structure, a foam type or a fiber-basedstructure, or a structure in which the active material is consolidatedby a polymer.

At present, five types of separator are used in such storage cells.

The first type comprises a corrugated or ribbed microporous sheet. Thisbulky separator requires a certain distance between electrodes and thuslimits cell capacity per unit volume. It is also observed that when theelectrolyte level falls, the operation of the cell is affected.

The second type comprises firstly a ribbed microporous sheet whoseribbed face is applied against the positive electrode, and secondly acomplementary felt adjacent to the plane face. This type suffers fromthe same drawbacks as a separator of the first type. In addition, it isthe seat of pockets of gas accumulation between the electrodes which isdetrimental to operation thereof.

The third type is constituted by a set of equidistant cylindrical orprismatic bars made of polyolefin and having a thickness that determinesthe spacing between the electrodes. This spacing is generally in therange 0.8 mm to 1.2 mm. Such a separator still has the drawbacks of aseparator of the first type, but in addition it does not constitute aneffective inter-electrode barrier against short circuits if theinter-electrode distance is less than 1 mm.

The fourth type as described in French patent number FR-A-2 628 892,which comprises:

two sheets of polyamide, polypropylene or polyethylene felt having athickness lying in the range 0.1 mm to 0.4 mm, and having a weight lyingin the range 10 g/m² 100 g/m² ; and

a membrane of cellophane or polyolefin interposed between these twofelts, and having a thickness lying in the range 0.02 mm to 0.03 mm.

The object of the membrane is to act as a barrier to oxygen diffusion,but not to short circuits.

The fifth type of separator is constituted by a corrugated andperforated sheet of PVC. It suffers from the same drawbacks as the firsttype of separator and in addition it does not provide the electrodeswith sufficient protection against short circuits.

An object of the present invention is to implement a separator enablingthe inter-electrode distance to be reduced, i.e. enabling cell capacityper unit volume to be increased, while nevertheless forming an effectivebarrier against short circuits and preventing the formation of pocketsof gas.

The present invention provides a separator for an alkaline storage cell,in particular for a nickel-cadmium storage cell, the separator beingconstituted by two sheets of polypropylene, polyamide, or polyethylenefelt having a thickness lying in the range 1.0 mm and 0.4 mm, and havinga weight lying in the range 10 g/m² to 100 g/m², together with anintermediate sheet interposed between said two felts, the separatorbeing characterized by the fact that said intermediate sheet is plane,has a thickness lying in the range 0.1 mm to 0.4 mm, is made of asintered material selected from polyvinyl chloride, polyethylene and acopolymer based on polyvinyl chloride or on polyethylene, has a porosityof not less than 30%, has pores of a diameter lying in the range 10 μmto 100 μm, and has a weight lying in the range 100 g/m² to 400 g/m².

Preferably, said weight is about 220 g/m² and said pore diameter lies inthe range 20 μm to 30 μm.

The separator may be assembled around electrodes in the form of bondedpockets; it may also be in the form of continuous strip disposedconcertina-like between the electrodes.

Other characteristics and advantages of the present invention appearfrom the following description of embodiments given by way ofnon-limiting illustrative example.

In the accompanying drawing:

FIG. 1 is a highly diagrammatic fragmentary section through a prior artseparator corresponding to the fourth above-mentioned type; and

FIG. 2 is a highly diagrammatic fragmentary section through a separatorof the invention.

The separator of FIG. 1 comprises two sheets 1 and 2 of polyethylenefelt having a thickness of 0.15 mm and a weight of 70 g/m², togetherwith an interposed sheet of cellophane 3 that is 0.02 mm thick.

The separator of FIG. 2 comprises a polyamide felt 10 having a thicknessof 0.15 mm and a polypropylene felt 20 having a thickness of 0.1 mm. Aplane intermediate sheet 4 is constituted by a microporous structurehaving a thickness of 0.2 mm and made by sintering a copolymer based onpolyvinyl chloride. The weight of this sheet is 220 g/m², its porosityis greater than 30%, and its pore diameter is about 20 μm to 30 μm.

The polyvinyl chloride may be replaced by polyethylene.

A nickel-cadmium storage cell including prior art separators has alifetime of 1,000 hours to 2,000 hours when permanently overcharged at40° C., and with an overcharging current density equal to one-tenth ofthe capacity of the storage cell. Using a separator of the inventionmakes it possible to increase the lifetime of the storage cell by afactor of two or more, under the same conditions of overcharging.

Naturally, the invention is not limited to the embodiment describedabove. Without going beyond the scope of the invention, any means may bereplaced by equivalent means.

We claim:
 1. A separator for an alkaline storage cell, the separatorbeing constituted by two sheets of polypropylene, polyamide, orpolyethylene felt having a thickness lying in the range 0.1 mm to 0.4mm, and having a weight lying in the range 10 g/m² to 100 g/m², togetherwith an intermediate sheet interposed between said two felts, theseparator being characterized by the fact that said intermediate sheetis plane, has a thickness lying in the range 0.1 mm to 0.4 mm, is madeof a sintered polymeric material selected from the group consisting ofpolyvinyl chloride, polyethylene and a copolymer based on polyvinylchloride, has a porosity of not less than 30%, has pores of a diameterlying in the range of 10 μm to 100 μm, and has a weight lying in therange 100 g/m² to 400 g/m².
 2. A separator according to claim 1,characterized by the fact that the pore diameter of said sheet lies inthe range 20 μm to 30 μm, and the weight of said sheet is about 220g/m².
 3. A separator according to claim 1, characterized by the factthat it is assembled around the electrodes of the storage cell in theform of bonded pockets.
 4. A separator according to claim 1,characterized by the fact that it is in the form of a continuous stripdisposed concertina-like between the positive and negative electrodes ofthe cell.
 5. A separator according to claim 1, wherein said intermediatesheet is made of a sintered copolymer based on polyvinyl chloride.
 6. Aseparator according to claim 1, wherein said intermediate sheet is madeof a sintered polymeric material comprising polyvinyl chloride.
 7. Aseparator according to claim 1, wherein said intermediate sheet is madeof a sintered polymeric material comprising polyethylene.